This invention relates to electrochemical cells. More particularly, it relates to high discharge rate electrochemical cells operable in a reserve-activated mode.
Electrochemical cells employing active alkali metal anodes and electrolyte systems based on oxyhalides or thiohalides of the Group VA and Group VIA elements of the Periodic Table of Elements are well known in the art. Such cells generally make use of an alkali metal salt solute in the electrolyte solution to increase the electrical conductivity of the solution and decrease the internal resistance of the cell.
U.S. Pat. No. 4,012,564 issued to James Auborn and assigned to the assignee of the instant application discloses a number of electrolyte solute salts for use in cells comprising an alkali metal anode and a covalent oxyhalide or thiohalide electrolyte solvent. As taught therein, Lewis acid solutes are used in conjunction with a metal halide such as lithium chloride, that is, in conjunction with a Lewis base. When such electrolyte solutes comprise Lewis acids alone or in excess, a problem which arises is the corrosive attack of alkali metal anodes by the Lewis acid. French Pat. No. 1,583,804 discloses a lithium anode cell employing an electrolyte system comprising lithium tetrachloroaluminate dissolved in an oxyhalide solvent. As taught therein, the lithium tetrachloroaluminate is formed by reaction of the Lewis acid, aluminum chloride, with the Lewis base, lithium chloride, in the oxyhalide solvent system. The Lewis base is deliberately employed therein in excess to forestall the possibility of corrosive attack of the alkali metal anode by aluminum chloride.
It has thus generally been held in the prior art that in the construction of primary electrochemical cells employing an active alkali metal anode and an electrolyte comprising an electrochemically reducible inorganic oxyhalide or thiohalide electrolyte solvent and a solute contained therein, that (1) the solute be a highly dissociated salt in the given electrolyte solvent in order to provide sufficient electrical conductivity for operation of the cell, and (2) that the solute be of a material other than a Lewis acid, to prevent corrosion of the active anode material. During discharge of cells based on an alkali metal anode, and an oxyhalide or thiohalide electrolyte solvent with a dissolved neutral salt solute, oxidation of the active alkali metal anode results in the production of alkali metal cations in the electrolyte solution. Simultaneous electrochemical reduction of the oxyhalide or thiohalide electrolyte solvent at the cathode produces, among other reaction products, halide ions. The halide ions react rapidly with alkali metal ions already present in high concentration in solution. The resulting alkali metal halide precipitates from solution and alkali metal ions produced by oxidation of the anode material replace the ions removed from solution. This process begins immediately upon discharge of the cell. In cells employing high surface area porous catalytic cathodes, precipitation of insoluble products of the cell discharge reactions clogs the pores of the cathode, reducing the rates of migration of materials to and from the active catalytic cathode surface, eventually causing the cell to cease functioning by passivating the catalytic surface. The overall result is a cell whose discharge rate and capacity are limited. One means employed to overcome this limitation has been the incorporation into the cell of large amounts of porous catalytic cathode material, which unfortunately in turn limits the volume of reducible electrolyte material which can be contained in a given cell volume.